Atlantic salmon ( Salmo salar) and rainbow trout ( Oncorhynchus mykiss) digest and metabolize nutrients differently. Effects of water salinity and dietary starch level

Abstract

Atlantic salmon (average weight: 199 g, group size: 55) and rainbow trout (average weight: 565 g, group size: 25) of the same age, kept in freshwater (FW) or full-strength seawater (SW), were fed diets containing 7% (LCH) or 23% (HCH) precooked maize starch, replacing dietary protein. The fish were fed ad libitum in triplicate groups. Feed intake was monitored. The species appeared quite similar regarding thermal growth coefficient (TGC); 3.09% and 3.14% for the salmon and the trout, respectively, and retention of digestible protein; 45.9% and 43.9%, and energy; 45.7% and 46.9%, respectively. Rainbow trout showed higher digestibilities than Atlantic salmon, most pronounced with the HCH diet. Digestibilities were negatively affected by starch level in Atlantic salmon, less severe in rainbow trout. Hepatosomatic index was higher in rainbow trout than Atlantic salmon when fed the HCH diet. Liver lipid, liver glycogen, plasma glucose and plasma insulin were higher in rainbow trout than in Atlantic salmon at both starch levels. The difference was greatest with the HCH diet. In Atlantic salmon, the HCH diet elevated only liver glycogen. Specific activities of maltase and iso-maltase were substantially higher in the rainbow trout than in the Atlantic salmon. In seawater, Atlantic salmon showed higher TGC than in FW. Rainbow trout seemed unaffected by salinity. Salinity did not affect body composition significantly, except for ash, which was higher in SW than in FW in both species. Digestibility of protein, starch, freeze dry matter and energy was higher in FW than in SW. For starch digestibility, this was true for Atlantic salmon only. Retention of protein, but not energy, was higher in SW than FW. No significant effects of water salinity were observed with regards to condition factor, dress-out percentage, HSI, liver lipid, liver glycogen, plasma glucose or plasma insulin. The high starch diet reduced TGC in both species with no significant effect on FCR or body composition. Negative effects of starch level were observed on digestibility of macronutrients and energy in Atlantic salmon, but not in rainbow trout. Retention of digestible energy was not significantly affected by starch level, indicating that starch energy can replace protein energy. The results clearly showed stimulating effects of dietary starch on mucosal maltase, sucrase and lactase activities in both Atlantic salmon and rainbow trout, contrary to results of previous research. The response in leucine aminopeptidase showed adaptation according to protein level.